This invention relates to sizing agents for carbon fibers and more particularly to water-based emulsion type sizing agents for carbon fibers which are superior in emulsion stability, are capable of wetting carbon fibers well, providing both cohesiveness and smoothness to carbon fibers, and improving workability in wrapping and weaving processes and have no adverse effect on adhesiveness between fibers and matrix resin when a composite material is made.
Carbon fibers are becoming noted for their superior strength, elasticity and heat resistance. Carbon fibers are usually used in the form of filaments but the number of twists per unit length cannot be increased much because they are low in expandability and quickly wear and tear by bending. If the number of twists is increased excessively, unevenness due to fiber bundles appear on the surface and the product becomes less attractive as a compound material. Carbon fibers are therefore treated with a small number of twists during their production and manufacturing processes and this causes many problems such as occurrence of filament breakages, filament defects and fluffs caused by friction against guides and rollers and frequent bending during later processes to lower the filament quality, insufficient wetting of carbon fibers and matrix resin due to fluffs, generation of pores in the product to lower the mechanical strength of the compound material and adverse effects on the work environment due to fluffs.
In view of the problems described above, sizing agents have been used to treat carbon fibers not only to improve their cohesiveness but also to cover and protect the surface of carbon fibers to reduce fluffs and filament breakages and to improve workability. Although various compounds are selected as sizing agent, depending on the matrix resin, use is generally made of epoxy resins, phenol resins, polyimides, polysulfones, etc. dissolved in an organic solvent such as acetone, methylethylketone, ethyl acetate, dichloromethane, etc. and such solutions are applied uniformly. In order to improve productivity of carbon fibers and to speed up and reduce energy consumption in the later manufacturing processes, however, use of highly flammable organic solvents is industrially disadvantageous from the safety and hygenical points of view.
For this reason, water-based emulsion type sizing agents are beginning to be applied recently. If the matrix resin is an epoxy resin, bis phenol A diglycidylethers or aromatic nitrogen-containing compounds are used but epoxy resins are generally hydrophobic and highly viscous such that they cannot easily be made into a stable emulsion. Moreover, although anionic surface active agents with a sodium sulfonate radical are appropriate as a surface active agent which can also be used as an emulsifying agent because of their emulsifying characteristics, it is not desirable to have an alkali metal contained from the point of view of heat resistance characteristics. This means that non-ionic surface active agents should be used as the emulsifying agent but, if an epoxy resin is emulsified only with ordinary non-ionic surface active agent, the long-term stability of the emulsion is generally inferior. If a previously considered water-based epoxy resin emulsion with an epoxy resin and a surface active agent is used to process carbon fibers, there are significant improvements in cohesiveness and adhesiveness to the matrix resin but since the coefficient of surface friction is high and the result is extremely poor in lubricity, filament breakages and fluffs occur by all kinds of contacts and frictions between fibers as well as between fibers and a metal during the weaving process prior to the production of prepregsheet. This is because epoxy resins are poor in lubricity and aromatic epoxy compounds, in particular, have very high coefficients of friction such that they are usable acceptably even as adhesives, and also because the aromatic surface active agents which are often used for emulsifying them have the effect of reducing lubricity.
In view of the above, there have recently been new attempts to improve the lubricity of carbon fibers which have been subjected to a sizing treatment. There has been a report on the use of surface active agents of polyoxyalkylene alkylether and polyoxyalkylene aliphatic acid ether types having a long-chain aliphatic groups for emulsifying and dispersing epoxy resins and another (Japanese Patent Publication Tokko No. 58-43513) on directly using a lubricant such as esters of stearic acid and silicone resins together with a sizing agent. The former method has problems in emulsifying and dispersing epoxy resins, however, while the latter method, although capable of improving lubricity, never fails to adversely affect the adhesiveness to the matrix resin if a lubricant is simply used together.
In summary, there has been no sizing agent for carbon fibers which is satisfactory for practical use from the points of view of adhesiveness, emulsifying and dispersion characteristics, lubricity, etc.